There is no way to know for sure. He will be allowed to write you, so ask him why he's there and for how long. I was in the SHU for one day, every minute felt like it was an hour - it is hell. I had my DHO hearing and was luckily released back into gen pop. There was another guy in there for eight months for having a cell phone. Rumor was he got a year in solitary and then was shipped to a cold climate to do the rest of his sentence.
Thank you for trying AMP!
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